One embodiment of the invention relates to a trench transistor. The development of new generations of DMOS power transistors, in particular of trench transistors, is driven by the reduction of the on resistivity Ron·A. Since controlled breakdown properties and a high avalanche strength are also desirable besides the low Ron·A, it is necessary to optimize the transistor cells but also the chip edge of trench transistors, in particular of dense trench transistors. Dense trench transistors are distinguished by such a narrow mesa zone that the breakdown location lies in the trench bottom region.
DE 102 07 309 describes a dense trench transistor having a breakdown zone at the trench bottom, which extends along the entire trench strip. Charge carriers generated there lead to a voltage drop along the body zone in the case of an electrical breakdown on account of a breakdown current. The body zone constitutes the base of a parasitic bipolar transistor, the emitter of which is formed by the source zones and the collector of which is formed by the drift zones. As the current density increases in breakdown operation, the voltage drop across the body zone reaches the order of magnitude of a diode forward voltage, so that the parasitic bipolar transistor turns on. The positive temperature coefficient of this transistor current leads to the destruction of the component as a result of overheating.
One known possibility for reducing the voltage drop across the body zone and thus counteracting the turning on of the parasitic bipolar transistor is afforded by a locally formed body reinforcement extending into the depth. In this case, the charge carriers generated along the bottom regions of the trench strips are extensively extracted, so that only a small voltage drop builds up along the body zone acting as parasitic base of the bipolar transistor. However, said body reinforcement extending into the depth takes up a large amount of space on account of its lateral variation during implantation and the subsequent lateral outdiffusion, so that this space is lost for the channel width. The capacitances formed in the region of the body reinforcement are likewise maintained, so that the figure of merit FOM (product of Ron·A·QGate/A where Ron is the on resistance, A is the transistor area and QGate/A is the gate charge QGate relative to the oxide transistor area A) assumes disadvantageous values.
A further possibility for improving the avalanche behavior of a dense trench transistor is described in DE 102 23 699. In this case, the trench and mesa width are varied along the trench strips in the dense trench region, so that the breakdown region can be defined at specific locations along the trench strips at the trench bottom. If this definition of the breakdown regions is combined with a corresponding positioning of the body contact zones, then it is possible, in breakdown operation/avalanche operation, for the charge carriers generated at the defined breakdown regions to be extracted without a voltage drop being built up along the body zone. In practice, however, it has been shown that a targeted modulation of the trench width and of the mesa width is difficult to implement since oxidations for rounding purposes are often used in the process sequence in order to avoid corners within the trench. The consequence of such oxidations is that predetermined trench width and mesa width modulations form only to a very small extent at the silicon surface and virtually do not form in the trench bottom region.